postit-debian: support POSTIT_GIT_TAG as a tag (not just a branch)

The previous override_dh_auto_build used 'git clone --branch <tag>'
which only accepts branch names. Feeding it 1.0.1-rc02 (a tag,
not yet pushed to origin) failed with 'La branche distante
1.0.1-rc02 n'a pas ete trouvee' and aborted the package build.

Switch to a 'git init + remote add + fetch --depth=1 origin <ref>
+ checkout FETCH_HEAD' sequence, which works for both branches
and tags, including local-only tags fetched via file:// (the
typical RC-staging workflow: 'git tag' on the local checkout,
then 'make POSTIT_GIT_URL=file:///... POSTIT_GIT_TAG=<tag>'
before pushing).

The fetch error now carries a diagnostic that points the operator
at the file:// override and at the missing 'git push'.

Adds tests/test-fetch-upstream.sh with four cases against the
real pazof/yavsc remote and a synthetic local repo: tag on
GitHub, branch on GitHub, .git reuse, and tag via file://.
Wired into 'make test'.

Makefile

@@ -4,6 +4,11 @@
 #   make deb      : build a .deb in the parent directory (or
 #                   $POSTIT_OUT_DIR if set). Equivalent to
 #                   dpkg-buildpackage -us -uc -b.
+#   make test     : run the non-regression tests under tests/.
+#                   The fetch test pins the upstream Git mechanics
+#                   that previously regressed on tag-vs-branch
+#                   handling (1.0.1-rc02). Set SKIP_NETWORK_TESTS=1
+#                   to skip the GitHub-egress parts.
 #   make clean    : rm -rf build/ and the per-package staging tree
 #                   under debian/postit.
 #   make source   : fetch the upstream yavsc source tree under
@@ -21,7 +26,7 @@ POSTIT_GIT_TAG ?= 1.0.0
 POSTIT_RUNTIME ?= linux-x64
 POSTIT_OUT_DIR ?= $(CURDIR)/..
 
-.PHONY: deb clean source
+.PHONY: deb clean source test
 
 deb:
 	@echo "  POSTIT_GIT_TAG=$(POSTIT_GIT_TAG)  POSTIT_RUNTIME=$(POSTIT_RUNTIME)  POSTIT_GIT_URL=$(POSTIT_GIT_URL)"
@@ -49,3 +54,6 @@ source:
 	mkdir -p build
 	git clone --depth=1 --branch $(POSTIT_GIT_TAG) \
 	    $(POSTIT_GIT_URL) build/yavsc-src
+
+test:
+	@bash tests/test-fetch-upstream.sh